"Flat product" is taken to mean any product, any material, or any object, whether solid, pasty or viscous, having one dimension called the thickness which is substantially less than the other two dimensions called the length and width. Preferably, but not exclusively, the flat products which the invention concerns are strips or sheets, in which the length exceeds the width, are continuous or discontinuous, moving or stationary, such as encountered in various industrial processes, and whose surface density or G.S.M., or the thickness for the same volume density is to be measured without contact.
By way of example, and non-limitingly, a flat product according to the invention may be:
a metal plate, sheet or thin sheet, such as encountered in metallurgy or metalworking PA1 a plastic film, of varying thickness, but often of relatively small thickness, for example approximately 10 microns PA1 a coating, with for example the support constituting fitted floor coverings PA1 a sheet of rubber PA1 a sheet of paper PA1 a fabric PA1 a pane of glass PA1 a board or panel of compressed wood particles. PA1 an emitter of an electromagnetic radiation or of a particle beam, whose source, situated on one side of the flat product to be measured, is directed toward the latter PA1 a receiver of the radiation or the beam, whose detector is situated with respect to the flat product to be measured, and in relation with the source, in order to receive a radiation or beam scattered or transmitted through said product, in attenuated form; this detector delivers a measurement signal PA1 optionally a mechanical support making it possible to support, and optionally to move the sensor or emitter/receiver assembly, with respect to the flat product being measured PA1 an analog and/or computerized member for processing the measurement signal, in order to obtain information representing the surface density or thickness of the flat product. PA1 the first, working in transmission, comprise a receiver whose detector is disposed on the other side of the flat product to be measured, with respect to the source, in order to receive the attenuated radiation transmitted through the flat product PA1 the second, working in back-scattering, comprise a receiver whose detector is disposed on the same side as the source, with respect to the flat product to be measured, in order to receive the attenuated radiation reflected by the flat product. PA1 emitters of an "X" or ".gamma." photon radiation, therefore with a wavelength generally lying between 5.times.10.sup.-4 and 5 nm PA1 and emitters of a so-called .beta. (positive or negative electrons), or .alpha. (helium nuclei) particle radiation. PA1 the measurement time and the emission rate of the source are two linked quantities; if N is the mean number of particles emitted per unit time, and T the measurement time, the total quantity of particles emitted per unit time is NT; since the emission of a particle is of "all or nothing" nature, the standard deviation at this mean is calculated from a binomial law, and is therefore equal to .sqroot.NT; the precision of the measurement is therefore a function of this parameter .sqroot.NT PA1 the other parameters-thickness of the flat product, nature of the flat product, nature of the emitter--affect the precision of the measurement, by virtue of the ability of the flat product to absorb the incident flux. PA1 the lifetime, represented by the radioactive half-life; a relatively long half-life is necessary for an industrial application (at least one year); a very large number of radioisotopes have half-lives ranging from a few milliseconds to a few minutes, hours or days PA1 it is necessary to be able to find the sources in the form of a sealed radioisotope source PA1 the activity of the source must be sufficient; sources of a few MBq are unusable, for example. PA1 the energy loss of a .beta.-ray for a given path is distributed around a mean value PA1 there is a very high dispersion in the paths of the .beta.-radiation coming from the source, which has strong repercussions on the energy loss PA1 the angular dispersion of the .beta.-radiation also leads to the number of .beta.-rays entering the detector also being random, with a dispersion around the mean which is absolutely non-negligible. PA1 it is necessary to obtain official authorization for possession of radioactive sources or artificial radioactive isotopes PA1 the radiation gages must be approved PA1 they must be used with strict safety and protection conditions PA1 etc. PA1 those comprising a detector consisting of an ionization chamber PA1 and those comprising a photomultiplier. PA1 the ionization chambers must be prepared meticulously; in particular, care must be taken to desorb the metal walls correctly before filling with the suitable gas; this requires handling operations which lead to extra cost; PA1 the second drawback stems from the fact that the migration of the ions in the gases is relatively slow; this has the consequence of limiting the response time of this type of detector; on the other hand, in the case of very high fluxes, the ions accumulate and the space charges which result therefrom can greatly limit the reliability of the measurement. PA1 in choosing for the X-radiation emitter, consisting of an X-ray tube, an anticathode which is specific, in the sense that its constituent material is of atomic number Z greater than or equal to 50, and preferably greater than 70, that it furthermore has good thermal conductivity and a high melting point; these constraints, added to the requirement of using a material which is available on the market, limit the possibilities and practice to tungsten (Z=74, thermal conductivity .rho.=1.77 Wcm.sup.-1 K.sup.-1, melting point m.p.=3410.degree. C.) and possibly to tantalum (Z=73, .rho.=0.574 Wcm.sup.-1 K.sup.-1, m.p.=2996.degree. C.), to rhenium (Z=75, .rho.=0.486 Wcm.sup.-1 K.sup.-1, m.p.=3180.degree. C.), to iridium (Z=77, .rho.=1.48 Wcm.sup.-1 K.sup.-1, m.p.=2410.degree. C.), to platinum (Z=78, .rho.=0.717 Wcm.sup.-1 K.sup.-1, m.p.=1772.degree. C.) and to gold (Z= 79, .rho.=3.19 Wcm.sup.-1 K.sup.1, m.p.=1064.degree. C.), PA1 and in providing a member for adjusting the voltage between the cathode and the anticathode of the X-ray tube, which member is designed to operate in a predetermined range of relatively low voltages (for example lying between 3.7 and 5 kV, in the case of measuring thin plastic sheets, of the order 10 .mu.m), PA1 the whole making it possible to obtain an X-ray emission spectrum essentially consisting of bremsstrahlung. The obtained X-ray spectrum is therefore continuous. It is limited, on the high energy side, by the maximum kinetic energy of the electrons striking the target, that is to say by the accelerating voltage between cathode and anti-cathode. Although X-rays with very low energy are also emitted in principle, the spectrum is notable only beyond approximately 2.5 keV. The X-rays of lower energy are stopped either by the anticathode itself, or by the output window of the tube. This spectrum is free from the feature K.alpha. or K.beta. lines, which are excited only for much higher high voltages (of the order of 70 kV and more, in the case of a tungsten target).
In the sense of the present invention, a flat product may also be a layer of a given material, deposited on a flat support, and whose surface density or thickness it is desired to measure as before.
In general, contactless radiometric gages comprise:
The variations in the measurement signal are a function of the quantity of particles or photons detected per unit time, and the energy given up by said particles or said photons. The variations in the attenuation of the incident flux, corresponding to the transmitted or reflected flux, are correlated with variations in the thickness or G.S.M. of the flat product, and consequently the measurement signal makes it possible to measure the thickness or G.S.M. of said flat product.
Two types of gage have been proposed, and correspond to the preceding definition:
Back-scatter gages are predominantly used for measuring the thickness of a layer deposited on a support, or the thickness of a flat product which is only accessible on one face, for example for manufacturing reasons.